Method and system for tuning an rfid interrogator

ABSTRACT

A method, apparatus, and system for periodically measuring the level of ambient noise found on a radio frequency channel used by a radio frequency identification interrogator to read radio frequency identification tags. The measured level of ambient noise is then used to dynamically adjust a threshold value used to predict collisions on the channel.

TECHNICAL FIELD

An embodiment of the present invention relates generally to RadioFrequency Identification.

BACKGROUND

Radio Frequency Identification (RFID) systems are employed to identifyand thus track a wide array of objects. Some examples of objects thatuse RFID technology for identification and tracking are documents (i.e.,passports and drivers license), retail merchandise, portableelectronics, furniture, parts, pharmaceuticals, and shipping containers.The RFID systems comprise one or more RFID interrogators that readinformation stored in RFID tags and a computer for processing theinformation. The RFID tag is normally attached directly to an object oris placed inside packaging that contains the object. Whenever an RFIDtag is within range of an RFID interrogator, the RFID interrogator readsthe information encoded on the RFID tag.

Ambient or background noise, which includes RF signals from other tags,interrogators, and devices, can make it difficult or in some casesimpossible for an RFID interrogator to detect an RF reply signal from anRFID tag. To mitigate this problem, protocol parameters have beenestablished. The protocol parameters control the transmission of an RFreply signal by an RFID tag. The values for the different protocolparameters are determined by the RFID interrogator and transmitted tothe RFID tags, which then use the parameters to generate and transmit RFreply signals. Properly set, the protocol parameters will increase theprobability that an RFID interrogator will accurately read all RFID tagswithin range even in the presence of ambient noise. If the protocolparameters are not properly set, the RFID interrogator will fail to readsome or all of the RFID tags within the range of the RFID interrogator.Therefore, it is desirable for the RFID interrogator to be able todetermine proper values for the protocol parameters to be able to readall RFID tags within range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an example RFID system comprising an RFIDInterrogator and a plurality of RFID tags.

FIG. 2 is high-level functional diagram of an example RFID Interrogator.

FIG. 3 is high-level flow diagram illustrating an example method fordetermining the threshold value for an RF channel.

FIG. 4 is high-level flow diagram illustrating an example method fordetermining that a collision has occurred.

FIG. 5 is high-level flow diagram illustrating an example method fordetermining the availability of an RF channel.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of the present invention. However, it will beunderstood by those skilled in the art that the present invention may bepracticed without these details and that numerous variations ormodifications from the described embodiments are possible.

As shown in FIG. 1, an exemplary illustration of an RFID system 10comprises an RFID interrogator 15 (also referred to as an RFID tagreader or RFID reader), ambient noise 40, and multiple objects 20, 25,30, 35 each containing an RFID tag 50, 55, 60, 65. Each of the RFID tagscontains information that identifies the RFID tag and by association theobject. The information stored on each RFID tag may uniquely identifyeach RFID tag from all other RFID tags or the information may identifyan RFID tag as belonging to a certain group (e.g., a one-galloncontainer of milk). In some embodiments, RFID tags contain additionalinformation beyond what is necessary to uniquely identify a tag or toidentify a group. For example, the RFID tag may contain informationidentifying a unique one-gallon container of milk from all othercontainers of milk plus have additional information specifying thesupplier, lot number, and expiration date of the milk.

Continuing with FIG. 1, the RFID interrogator 15 is designed to read theinformation from each RFID tag 50, 55, 60, 65 that is within the RFIDinterrogator's 15 range 45. The RFID interrogator 15 reads informationstored on an RFID tag by transmitting a radio frequency (RF) signal.This signal is referred to as a query signal or query command. The querysignal is received by all RFID tags 50, 55, 60, 65 that are in range 45of the RFID interrogator 15. The RFID tags then transmit an RF replysignal that includes the information stored in the RFID tag. The RFIDinterrogator 15 receives the RF reply signals from each RFID tag andrecovers the information contained in each RF reply signal. Theinformation may contain almost any type of data including a globallyunique ID number, a price, tracking data, a destination, a part number,a serial number, or other attributes or combination of attributes thatdescribe the object associated with the tag. Some RFID systems supportRFID tags that contain relatively small amounts of information whileother systems support RFID tags that contain large amounts of data andsome support both types of tags.

To reduce cost, some types of RFID tags do not have an internal sourceof power, i.e., a battery, to drive the electronics of the tag. Thesetypes of tags are referred to as passive tags. A passive tag comprisesan antenna connected to electronics, which usually consist of a singleintegrated circuit (IC). Passive tags are powered by the minuteelectrical current induced in the tag's antenna by an incoming RF signaltransmitted by an RFID interrogator. The induced current provides asource of electrical energy that is suffient to power up the IC and totransmit a reply signal back to the RFID interrogator. Passive RFID tagsgenerate a RF reply signal using a backscattering technique where the RFsignal from the interrogator is modulated and reflected back to the RFIDinterrogator. Information stored in each tag is included in the RF replysignal. This transmission method reduces the power needed to operate atag thus eliminating the need for a battery and reducing the cost of thetag. However, the RF signal transmitted by a passive tag is very weak.

Most environments have some level of detectible ambient (or background)RF noise 40 that can interfere with communications between the RFIDinterrogator 15 and the RFID tags. The source or sources of the ambientRF noise 40 may reside inside or outside the range 45 of the RFIDinterrogator 15. Because the RF reply signals are very weak, the RFIDinterrogator may fail to distinguish between RF reply signals from tagsand ambient RF noise 40. When this occurs, the RFID interrogator 15 willfail to read one or more RFID tags. Additionally, the level of ambientRF noise 40 will vary over time and can cause intermittent failures andreliability issues.

Referring to FIG. 2, the interrogator 15 is comprised of a processor 110connected over a bus 120 to a memory 115, a communications interface125, and an RF interface 105. The memory 115 contains both volatile andnon-volatile types of memory. The non-volatile memory is used to storeinstructions that when executed by the processor 110, control theoperation of the RFID interrogator 15. The non-volatile memory alsocontains parameters that are used by the instructions to control theRFID interrogator 15. In some embodiments, the processor 110 has theability to change the contents of the non-volatile memory. In someembodiments, the processor 110 accesses the memory over a dedicatedmemory bus. The processor 110 uses the communications interface 125 tocommunicate with one or more external systems 145. The communicationsinterface may be a wired interface such as Ethernet or a wirelessinterface such as Wi-Fi (IEEE 802.11). In some embodiments, one of theexternal systems is a point-of-sale terminal used in a retailenvironment. In some embodiments, one of the external systems includes adatabase that is used with information from the RFID tags to identifyand track objects.

Continuing with FIG. 2, the processor 110 uses the RF Interface 105 tocommunicate with one or more RFID tags. The RF interface 105 comprisesan RF transmitter 140 and an RF receiver 135, both supporting multipleRF channels used to communicate with RFID tags. The RF transmitter 140and the RF receiver 135 are connected to antenna 130 and use the antenna130 to transmit and receive signals to and from RFID tags. In someembodiments, multiple antennae are used to increase the range andability to communicate with the RFID tags. Because a signal from apassive RFID tag is weak, objects placed between the RFID tag and theantenna 130 act to shield and prevent the weak RF reply signal fromreaching the antenna 130. An RFID system with more than one antennaincreases the probability that the RF reply signal from an RFID tag willbe received by at least one of the system's antennae. The additionalantennae thus increase the range 45 and reliability of the RFIDinterrogator 15.

The RFID interrogator 15 is limited to receiving only one RF replysignal, per RF channel, at a time. In a multi-tag environment wheremultiple RFID tags are in range 45 of the RFID interrogator 15 at anygiven time, a collision occurs if more than one RFID tag replies at thesame time and on the same RF channel. When a collision occurs, all datais lost because the signals are unintelligible. To prevent collisions,the RFID interrogator 15 must singulate each RFID tag within the RFIDinterrogator's 15 range 45. Singulating an RFID tag occurs when the RFIDinterrogator 15 is able to identify and communicate with only one RFIDtag at a time. The singulating process involves the RFID interrogator 15setting and passing protocol parameters to all RFID tags within range45. The RFID tags then use the parameters to determine the appropriatetime and channel to use when communicating with the interrogator 15. Ifthe parameters are properly set, the interrogator 15 will successfullysingulate all RFID tags within range 45. If the parameters are notproperly set, collisions will occur between RFID tags and singulationwill take longer or may not occur at all.

EPCglobal Inc™ is an international organization that has establishedvoluntary standards that govern certain aspects of an RFID system. Onestandard from this organization is the “EPC™ Radio-Frequency IdentityProtocols Class-1 Generation-2 UHF RFID Protocol for Communications at860 MHz-960 MHz, Version 1.0.9,” which is herein incorporated byreference, includes guidelines for the operation of the RFIDinterrogator and tags. An interrogator that meets the requirements ofthis standard is described as a class 1, generation 2 interrogator. RFIDinterrogator 15 complies with the standard for a class 1, generation 2interrogator although in other embodiments, RFID interrogator 15 willcomply with other RFID standards that work with passive and/or activeRFID tags.

The EPCglobal Inc™ standard for a class 1, generation 2 RFID systemdefines a set of protocol parameters that govern the performance andaccuracy of an RFID interrogator as it singulates RFID tags in an RFIDsystem. The protocol parameters include: 1) “Q” which sets the number ofslots in the round; 2) “DR” (TRcal divide ratio) sets the T=>R linkfrequency; 3) “SEL” chooses which Tags respond to the Query signal orcommand; 4) “SESSION” chooses a session for the inventory round; and 5)“TARGET” selects whether tags whose inventoried flag is A or Bparticipate in the inventory round. The purpose of the protocolparameters is to eliminate multiple simultaneous tag responses (i.e.,collisions) to a query signal from the RFID interrogator 15. Theprotocol parameters are broadcast to all tags within range of theinterrogator 15 during the query process.

The “Q” configurable protocol parameter identifies the number of timeslots available for the tags to reply to a query signal. Each tagrequires one time slot to reply to a query signal from the interrogator15. In a case where Q is set to 1, all tags within range of theinterrogator will transmit their reply to a query signal in the sametime slot. If multiple tags are in range of the interrogator, the tagswill all transmit their reply in the same time slot and cause acollision. To prevent any possibility of a collision, Q could be set tothe maximum value of 32,768 (2¹⁵). This would prevent collisions butthroughput performance of the RFID system would suffer greatly becauseit could take up to 32,768 time slots to singulate a tag. It istherefore desirable to use a smaller value for “Q” when the tagpopulation is small and a larger “Q” value when the tag population islarge to maximize throughput while reducing or eliminating collisions.

As shown above, when the protocol parameters are not properly set, acollision will occur when the interrogator 15 fails to successfullysingulate all the RFID tags causing more than one tag to respond at thesame time, on the same RF channel. The EPCglobal Inc™ standard does notprovide an intrinsic feature that will detect a collision; therefore theinterrogator 15 does not directly sense a collision or the presence ofmultiple tags during a collision. Additionally, a collision can bemistaken for a situation where there are no tags within range 45 of theinterrogator 15 when the query signal is transmitted, so theinterrogator 15 will have no reply signals to detect. Thus, aninterrogator that implements the class 1, generation 2 standard can notintrinsically distinguish between a collision that signifies thepresence of multiple tags and the absence of a response that signifiesno tags are present.

Architects of the standard sought to address the problems of systemperformance and distinguishing between no tags and multiple tags bysuggesting that the interrogator use a default setting for the protocolparameters that force a predetermined minimum number of time slots. Thepresumption being that the minimum number of time slots will be largeenough to allow some successful singulations to occur in a multi-tagenvironment but small enough so as not to adversely affect systemperformance. An algorithm is outlined in which a feedback loop is usedto increase or decrease the number of time slots based upon the numberof successful singulations. The algorithm states: 1) decrease the numberof time slots (but not below the minimum number) if the number ofsuccessful singulations is equal to zero; 2) keep the number of timeslots the same, if the number of successful singulations is equal toone; and 3) increase the number of time slots, if the number ofsuccessful singulations is greater than one. This algorithm would becontinuously applied while the RFID system is operating.

In addition to using the number of successful singulations to adjust thenumber of time slots, the architects of the standard sought to predictthe occurrence of a collision between tags. While the interrogator 15cannot intrinsically detect a collision between two or more tags, theinterrogator can compare the magnitude of a received signal to athreshold value and then predict whether a collision has occurred. Thearchitects also defined a statistically calculated static thresholdvalue for each RF channel used by the interrogator 15. (The thresholdvalue is sometimes referred to as a decision threshold.) Using thismethod, if a received signal exceeds the threshold value, a collision isassumed to have occurred. To prevent another collision, the number oftime slots is increased. If the received signal falls below thethreshold value, it is assumed that no collision has occurred. Toimprove system throughput performance, the number of time slots isdecreased when there are no collisions but not below the minimum number.

This scheme is prone to errors because the statistically defined staticthreshold values cannot adapt to the dynamic nature of the ambient RFnoise in an RFID environment. The level of ambient RF noise 40 variesover time and by geographical location. To address this issue, thestatistically defined static values are set relatively high. Inenvironments that have periods of low ambient RF noise 40, the thresholdvalues are too high causing the interrogator 15 to miss a weak responsefrom a tag at the edge of the interrogator's 15 range 45. In this case,the interrogator 15 falsely sees the weak response as noise. Inenvironments that have periods of high ambient RF noise 40, thethreshold values are too low causing the interrogator 15 to falselyidentify ambient RF noise 40 as collisions. The false collisions willcause the RFID system to increase the number of time slots as itattempts to reduce the number of collisions. This causes intermittentproblems that are difficult and expensive to diagnose. In environmentsthat have a constant high level of ambient RF noise 40, the increase incollisions will adversely affect the performance of the RFID system andthe interrogator's ability to read tags.

Referring now to FIG. 3, there is provided a block diagram thatillustrates a method for dynamically adjusting the interrogator's 15threshold values used to predict collisions. The interrogator 15communicates with the RFID tags on a number of different radiofrequencies or RF channels and a threshold value is maintained for eachRF channel. In step 300, the interrogator 15 selects one of the RFchannels supported by the RF receiver 135. In some embodiments, it isalso possible to adjust the bandwidth of the RF receiver 135. In whichcase, the bandwidth is narrowed to increase the sensitivity of the RFreceiver 135. In step 305, the interrogator 15 causes the RF receiver135 to capture a sample of the signal being received on the selected RFchannel. This signal represents the instantaneous ambient RF noise forthe selected RF channel. The interrogator 15 then performs a spectralanalysis on the sampled signal 310. This is accomplished in thefrequency domain using Fourier analysis. However, other methods can beused to perform the same analysis. The spectral analysis produces aresult that is the magnitude of the instantaneous ambient RF noisesignal for the selected RF channel. In step 315, the interrogator 15stores the result in a circular buffer that is dedicated to storingresults for the selected RF channel. During idle periods, theinterrogator 15 continuously repeats this process for each RF channel.At some point, the circular buffers for each RF channel become full. Atwhich time, the oldest result is removed and the newest result is added.The size of the circular buffer is selectable.

Concurrent with determining the magnitude of the instantaneous ambientRF noise for each RF channel, the interrogator 15 calculates a dynamicthreshold value for each channel by using the results stored in thecircular buffer assigned to the RF channel. This is illustrated asfollows. In step 350, the interrogator 15 selects a single RF channel.In step 355, the interrogator 15 reads all of the results from thecircular buffer assigned to the selected RF channel. The ambient RFnoise for the channel is modeled using a statistical Gaussiandistribution, which has well known formulas to calculate the mean,variance, and standard deviation. Assuming the ambient noise willcontinue to follow a Gaussian distribution model, the interrogator 15sets the threshold value for the selected channel to the mean of theresults from the circular buffer plus three standard deviations 360.During idle periods, the interrogator 15 periodically repeats thisprocess for each channel. In this way, the threshold value for eachchannel is periodically updated and based on a statistical analysis ofactual real-time ambient RF noise found on each RF channel.

Now referring to FIG. 4, using the above method for determining theinstantaneous ambient RF noise on an RF channel and the threshold valuefor a RF channel, the interrogator 15 can predict whether a collisionhas occurred. The method begins when the interrogator 15 receives asignal in response to a query signal but the received signal isunintelligible (step 400). The interrogator 15 performs the spectralanalysis on the received signal and determines a magnitude for thereceived signal (step 405). The magnitude is then compared to thethreshold value for the RF channel used to receive the signal (step410). The interrogator 15 concludes that a collision has occurred whenthe magnitude of the signal is greater than the threshold value (step415). When the magnitude of the signal is less than the threshold, theinterrogator 15 concludes that no collision has occurred and that thereare no tags are within range (step 420).

When a collision is predicted to have occurred, the interrogator 15adjusts the protocol parameters to increase the number of time slots inan effort to eliminate future collisions. The interrogator 15 thenissues another query signal and checks the replies. This process isrepeated until there are no collisions and all tags are read. Predictingcollisions using dynamic threshold values is more accurate anddependable than using statistically defined static threshold values.Therefore, the use of conservative protocol parameters, that force aminimum number of time slots that adversely affects system throughput,is no longer required. The conservative protocol parameters were anadditional safeguard that is only required when the interrogator 15 isusing statistically defined static threshold values. Therefore, usingdynamic threshold values increases prediction accuracy and allows theinterrogator 15 to initially set the protocol parameters to as few asone time slot to maximize system throughput while still effectivelyhandling collisions.

It will be appreciated that the above method of dynamically determiningthe ambient RF noise of an RF channel may also be used to implement alisten-before-talk (LBT) strategy in an RFID system as illustrated inFIG. 5. In this strategy, an RF channel is selected (step 500) but priorto using the RF channel, the interrogator 15 takes a sample of theinstantaneous ambient RF noise on the RF channel (step 505). Theinterrogator 15 then determines a magnitude for the instantaneousambient RF noise using spectral analysis (step 510). When the magnitudeof the instantaneous ambient RF noise exceeds the threshold value forthe selected RF channel, the interrogator 15 concludes that the RFchannel is not available and selects another RF channel (step 520). Whenthe magnitude is less than the threshold value, the interrogator 15concludes that the RF channel is available for use (step 525).

There are many sources of the instantaneous ambient RF noise and theywill vary from one environment to another. Some examples of sources are:other interrogators, a communication system operating nearby (i.e., WiFior cellular), spurious emissions from electronic equipment, or acombination of sources. Whatever the source, the use of dynamicthreshold values by the interrogator 15 will allow the RFID system todynamically adapt to its current environment so as to maintain maximumthroughput and reliability.

It should also be appreciated that an RFID system using active RFID tags(tags that use batteries to power the electronics and RF transmitter) ora combination of active and passive RFID tags will have the samebenefits as the above RFID system.

While the present invention is disclosed in the context of a presentlypreferred embodiment, it will be recognized that a wide variety ofimplementations may be employed by a person of ordinary skill in the artconsistent with the above discussion, the drawings, and the claims thatfollow below.

1. A method for use in optimizing a radio frequency identificationinterrogator, the method comprising: receiving a signal within a radiofrequency channel; performing spectral analysis on the signal to obtaina result; and setting a decision threshold for the interrogator based ona statistical analysis that uses the result.
 2. The method of claim 1,wherein the signal comprises ambient noise on the radio frequencychannel.
 3. The method of claim 1, wherein the result produced by thespectral analysis is the magnitude of the signal.
 4. The method of claim1, wherein the spectral analysis comprises a Fourier analysis of thesignal.
 5. The method of claim 1, wherein the result is stored in acircular buffer assigned to the radio frequency channel.
 6. The methodof claim 5, wherein the statistical analysis includes using a Gaussiandistribution model on data stored in the circular buffer.
 7. The methodof claim 6, wherein the decision threshold is set to the mean plus threestandard deviations.
 8. The method of claim 7, wherein the decisionthreshold is associated with the radio frequency channel.
 9. A radiofrequency identification interrogator comprising: a radio frequencyreceiver adapted to receive signals on a radio frequency channel; aprocessing unit in communication with the receiver; and a memory unitelectrically coupled to the processing unit, wherein the memory devicehas stored therein a plurality of instructions which, when executed bythe processing unit, cause the processing unit to: (i) receive a signalfrom the receiver; (ii) perform a spectral analysis on the signal toobtain a result; and (iii) set a decision threshold for the interrogatorbased on a statistical analysis that uses the result.
 10. Theinterrogator of claim 9, wherein the signal comprises ambient noise onthe radio frequency channel.
 11. The interrogator of claim 9, whereinthe result produced by the spectral analysis is the magnitude of thesignal.
 12. The interrogator of claim 9, wherein the spectral analysiscomprises a Fourier analysis of the signal.
 13. The interrogator ofclaim 9, wherein the result is stored in a circular buffer assigned tothe radio frequency channel.
 14. The interrogator of claim 13, whereinthe statistical analysis includes using a Gaussian distribution model ondata stored in the circular buffer.
 15. The interrogator of claim 14,wherein the decision threshold is set to the mean plus three standarddeviations.
 16. The interrogator of claim 15, wherein the decisionthreshold is associated with the radio frequency channel.
 17. A radiofrequency identification system, the system comprising: a computer; aradio frequency identification interrogator in communication with thecomputer, the interrogator comprising: a radio frequency receiveradapted to receive signals on a radio frequency channel; a processingunit in communication with the receiver; and a memory unit electricallycoupled to the processing unit, wherein the memory device has storedtherein a plurality of instructions which, when executed by theprocessing unit, cause the processing unit to: (i) receive a signal fromthe receiver; (ii) perform a spectral analysis on the signal to obtain aresult; and (iii) set a decision threshold for the interrogator based ona statistical analysis that uses the result.
 18. The interrogator ofclaim 17, wherein the signal comprises ambient noise on the radiofrequency channel.
 19. The interrogator of claim 17, wherein the resultproduced by the spectral analysis is the magnitude of the signal. 20.The interrogator of claim 17, wherein the spectral analysis comprises aFourier analysis of the signal.
 21. The interrogator of claim 17,wherein the result is stored in a circular buffer assigned to the radiofrequency channel.
 22. The interrogator of claim 21, wherein thestatistical analysis includes using a Gaussian distribution model ondata stored in the circular buffer.
 23. The interrogator of claim 22,wherein the decision threshold is set to the mean plus three standarddeviations.
 24. The interrogator of claim 23, wherein the decisionthreshold is associated with the radio frequency channel.
 25. A methodfor use in a radio frequency identification interrogator using alisten-before-talk strategy, the method comprising: selecting a radiofrequency channel; receiving a signal within the radio frequencychannel; performing spectral analysis on the signal to obtain a result;and selecting a different radio frequency channel when the result isgreater than a decision threshold.
 26. The interrogator of claim 25,wherein the signal comprises ambient noise on the radio frequencychannel.
 27. The interrogator of claim 25, wherein the result producedby the spectral analysis is the magnitude of the signal.
 28. Theinterrogator of claim 25, wherein the spectral analysis comprises aFourier analysis of the signal.